how do I render my animation and the rest of the scene seperately
You can render the first static frame, save it as image and then plug into Alpha Over node in compositing.

Disable animated river. Uncheck Disable in Renders icon in the Outliner tab: 
Render only Statics and save on disk: 
Enable the river in Render and disable all statics (same as 1)
Check Fill&gt;Transparent (available for both Eevee and Cycles):
Set up compositing Alpha Over node:
Render the river. Compositing will be executed for each frame of the animation: